When a fluid enters a rotating pipe, a swirl boundary layer with thickness of δ̃S appears at the wall and interacts with the axial momentum boundary layer with thickness of δ̃. The swirl is produced by the wall shear stress and not due to kinematic reasons as by a turbomachine. In the center of the pipe, the fluid is swirl-free and is accelerated due to axial boundary layer growth. Below a critical flow number φ < φc, there is flow separation, known in the turbomachinery context as part load recirculation. The previous work analyzes the flow at the inlet of a coaxial rotating circular pipe (R̃=R̃0). For a systematic approach to a turbomachine, the influence of the turbine's and pump's function, schematically fulfilled by a diffuser and a nozzle, on the evolution of the swirl and flow separation is to analyze. The radius of the rotating pipe depends linearly on the axial coordinate, yielding a rotating circular diffuser or nozzle. The swirl evolution depends on the Reynolds number, flow number, axial coordinate, and apex angle. The influence of the latter is the paper's main task. The circumferential velocity component is measured applying one-dimensional laser Doppler anemometry (LDA) to investigate the swirl evolution.

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